The speed of Metal-Oxide-Semiconductor (MOS) transistors is closely related to the drive currents of the MOS transistors, which drive currents are further closely related to the mobility of charges. For example, NMOS transistors have high drive currents when the electron mobility in their channel regions is high, while PMOS transistors have high drive currents when the hole mobility in their channel regions is high.
Compound semiconductor materials of group III and group V elements (known as III-V compound semiconductors) are good candidates for forming transistors due to their high electron mobility. Therefore, transistors formed on III-V compound semiconductors have been explored. III-V compound semiconductor films, however, need to be grown on other substrates because it is difficult to obtain bulk III-V crystals. The growth of III-V compound semiconductor films on dissimilar substrates faces difficulties because these substrates have lattice constants and thermal expansion coefficients different than that of the III-V compound semiconductors. Various methods have been used to form high-quality III-V compound semiconductors that do not suffer from severe defects. For example, III-V compound semiconductors were grown from trenches between shallow trench isolation regions to reduce the number of threading dislocations.
The formation of III-V compound semiconductors from trenches typically includes an epitaxy growth, followed by a Chemical Mechanical Polish (CMP) to remove excess III-V compound semiconductors over the shallow trench isolation regions. By forming the III-V compound semiconductors, some defects may be eliminated. The eliminated defects, however are the defects that are not vertically grown. Hence, with the growth of the III-V compound semiconductors, the defects are also grown, and hence extend to, and are blocked by, the sidewalls of the STI regions. These defects include stacking faults and threading dislocations. Other types of defects such as anti-phase domain defects, however, may grow vertically, and hence will not be blocked by shallow trench isolation regions. These defects hence cannot be eliminated by the re-growth process. Anti-phase domain defects are the defects that occur when a compound semiconductor is grown. If a single-element semiconductor such as silicon or germanium is grown, then the anti-phase domain defects will not occur.